The present invention relates to a drive circuit for applying power to a capacitive load, such as an electroluminescent device, typically an electroluminescent panel ("EL panel"). EL panels are capable of emitting light of rather uniformly diffused illumination from an area defined by a major surface of the respective device or EL panel. Electroluminescent panels are desirable backlighting sources for currently widely used and well known LCD displays. LCD displays are popular for use in portable apparatus because of their small size and low power consumption. However, advantages of low power consumption by the LCD displays may be totally offset by special power requirements for driving EL panels to provide backlighting for the displays under poor external lighting conditions.
Power management for backlighting EL panels becomes particularly critical when an LCD display is that of a portable data terminal or of a laptop or palmtop computer. These devices typically have LCD screens which are typically of a size of several square inches, requiring power to illuminate these comparatively large areas. In addition, use periods of the terminals or computers are extensive. A relatively larger screen size is generally necessitated by the need to display more information, hence requiring more time of an operator to work with the displayed information. Increased power needs for backlighting because of a larger screen size is therefore often coupled to further power requirements to satisfy longer operating periods. Portable data terminals are desirably designed for four to eight hours of continuous work without intervening power recharge requirements.
EL panels require AC (alternating current) power sources which operate at specified voltages and within an optimum frequency range. These voltages and frequencies are typically specified by the manufacturers of the panels. For example, a panel may be driven by typical line power, operating at 60 hz. Different manufacturers may recommend various higher driving frequencies for their respective panels, for example 100 to 1000 Hz, and voltages in a range of 75 to 200 volts RMS may be employed for an increased light output from the panels. These optimum frequencies and voltages are typically not readily available in portable electronic equipment.
A well known manner for applying power to an EL panel is through a secondary output of an autotransformer, coupled in series with the EL panel. The inductance of the autotransformer is configured with respect to the capacitance of the EL panel to have a resonant frequency at the desired operating AC frequency of the EL panel. The input drive circuit is then configured to drive the autotransformer at the resonant frequency of the autotransformer output circuit including the load element of the EL panel. The resonant autotransformer drive method is generally quite power efficient. Energy losses in the transformer and drive circuit and power consumption of the EL panel are supplied through the primary drive of the autotransformer. However, a significant disadvantage of the autotransformer is its physical size. For the resonant frequency to be the desired operating frequency for the EL panel, the inductance and hence the core size of the autotransformer is large in comparison with the physical size of the EL panel. In addition, the magnitude of the desired inductance requires a substantial number of turns of wire as the windings for the autotransformer. For most portable devices, the resulting size and weight increases are most undesirable. In addition, the components for the resonant circuit need to be carefully matched to the capacitance of the particular EL panel. Other arrangements for providing power to EL panels appear to be therefore desirable.
U.S. Pat. No. 5,027,040 discloses a switching invertor circuit supplying power to an EL panel from a dual mode DC (direct current) power supply. An inductor is coupled in series with the EL panel and is chosen with an inductance to cause the time resulting resonant frequency to be one-half of the desired operating frequency of the EL panel. For the second or negative driven half of the desired power cycle of the EL panel, the switching invertor circuit inverts the applied driving voltage. The referred to '040 patent discloses a synchronous switching operation of switching elements in the invertor circuit. Switching preferably takes place during periods of current reversal and, ideally, during conditions of zero current flow through the switching elements to minimize switching losses.
It should be realized that both of the above examples of the state of the art provide drives which are tuned to the capacitance of the EL panel to operate most efficiently. In view of the above, improved power circuits without a tuned inductor and including other improvements in panel illumination controls would greatly enhance the usefulness of EL panels for illuminating LCD displays.